Considering either a new X-power buffer spring or heavier buffer weight or both. What do you use?

Neither.

It was designed for a standard spring and a standard buffer.

Is it not functioning properly? Details?

The .458S is NOT a 5.56 so take everything you think you know about a 5.56 and forget it.  As said above, it was designed to work with standard buffers and springs, rifle or CAR, and of the dozen .458 S I own, only the shorties with barrels under 10” do I use a HB and never a XP spring or any buffer heavier than an H.

If you have a knock-off barrel and bolt though, all bets are off.  Some of these cheap barrel makers have things so mucked up you cannot get them to work properly even if you cut off a chicken’s at midnight on a full moon.

In short, always start with standard buffer and spring then adjust if needed.  On a quality barrel and bolt, you won’t need to adjust anything.

Was just trying to possibly reduce felt recoil on the old shoulder. Might just have to go lighter with reloads.

Muzzle brake is your best bet.

And stay away from 405s. Those are probably the most intense on the shoulder.

Recoil pads are wonderful.

TROMIX 4 port fish gill brake, Magpul CTR stock, Limbsaver CTR recoil pad.  Those three items will tame the recoil a heck of a lot, even with supersonic 500 gr. loads.